Cryptorchidism in Horses

Cryptorchidism is a congenital condition (i.e., the individual is born with it) in which one (unilateral) or both (bilateral) of the testes fail to descend into the scrotum. A number of common terms have been used to describe cryptorchids, including "rig," "false rig," and "ridgling." Embryologically, the testes develop high in the abdomen near the kidneys. During fetal development they migrate down through the abdomen, passing through the inguinal rings, and eventually reside in the scrotum, usually by the 9th to 11th month of gestation.

When the testis and epididymis (the duct that stores sperm as it matures) are retained within the abdomen, the term abdominal cryptorchid is used. When the tail of the epididymis and/or variable amounts of the deferent duct (runs from the epididymis to the urethra) extend into the inguinal canal (allows passage of the spermatic cord), it's termed an abdominal or abdomino-inguinal cryptorchid. When the testis is in the inguinal canal, the term inguinal cryptorchid or the more vernacular "high flanker" is used.

The testes are maintained in the scrotum at a lower temperature than the rest of the body's organs. This is important because in order for normal spermatogenesis to occur, the testes must be 4-6°C (5-9°F) below core body temperature. Therefore, cryptorchid testes cannot produce sperm normally, and a bilateral abdominal cryptorchid would be sterile.

Diagnostics

Diagnosis of cryptorchidism begins with reviewing the horse's history and performing a physical examination that includes attempts to locate and identify the retained testis using palpation (while under sedation), transrectal palpation, and ultrasonography. Identification of the retained testis by palpation can be difficult because these testes are often smaller and softer than normal testes, and they can be confused with other structures.

The most definitive method of determining if a cryptorchid testis is present is via hormonal assay. It is usually beneficial to perform an hCG (human chorionic gonadotropin, a hormone that will stimulate any testicular tissue to release androgens) stimulation test in addition. A blood sample for baseline testosterone is obtained, then the horse is administered hCG, and a second blood sample is obtained within 24 hours for comparison; a twofold to threefold increase in testosterone can indicate a cryptorchid. Another hormone, estrone sulfate, can be similarly assayed for this purpose.

Retained testes should be removed, not only because this helps eliminate objectionable behavior, but also because retained testes are more prone to spermatic cord torsion and have a higher incidence of developing tumors. The surgical approach used to remove the cryptorchid testis depends on its location and the surgeon's preference. Laparoscopic surgery to remove abdominally retained testes can minimize the required recovery time.

While there are anecdotal reports of success of hormonal treatments for inducing the descent of retained testes, there are no controlled studies documenting the efficacy of these treatments. Some cryptorchid colts have a testis that was inguinally retained miraculously appear in the scrotum after two to three years without any treatment. It is virtually impossible for abdominally retained testes to accomplish this, however. After birth, the internal inguinal ring reduces in size so that if the testis has not passed through it and out of the abdomen by two weeks of age, it will be permanently retained in the abdomen.

Heritability

While most veterinarians and researchers agree there is a genetic component to the condition, the mode of transmission has not been established. Just because a stallion is a cryptorchid doesn't mean that his male offspring will necessarily be cryptorchids, nor does the absence of cryptorchidism in the sire guarantee that all of his male offspring will be free of the condition.

The relative risk of cryptorchidism appears to be breed-dependent. In one study of 5,009 cryptorchidism cases, Thoroughbreds, Standardbreds, Morgans, Tennessee Walking Horses, and Arabians had cryptorchidism diagnosed less frequently than expected, whereas Percherons, American Saddle Horses, Quarter Horses, and ponies as a group were overrepresented. Veterinarians hope that with advancements in equine genomics, the heritability of cryptorchidism will be more clearly elucidated.

About the Author

Steven P. Brinsko, DVM, MS, PhD, Dipl. ACT, an Associate Professor and Chief of Theriogenology at Texas A&M University, received his DVM from the University of Florida, MS from Texas A&M University, and PhD from Cornell University; he is also a board certified specialist in the American College of Theriogenologists (Veterinary Reproduction). His interests include all areas of equine reproduction with emphasis on the stallion. Dr. Brinsko has given numerous presentations at national and international meetings and has authored or co-authored over 100 scientific publications as well as 11 book chapters related to the field of reproduction.

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